Ukrainian authorities say between 12 and 14 soldiers, including an army general, were killed when pro-Russian separatists shot down a military helicopter near Slovyansk, while Russia urged the West to press Kyiv to stop the military operation and avert a "catastrophe."

Oleksandr Turchynov told parliament on May 29 that the helicopter was brought down using "a Russian portable antiaircraft missile," killing 14 people.

Ukraine's volunteer National Guard later revised the death toll down to 12 and said one person was heavily injured. It confirmed that General Serhiy Kulchytskiy, who was in charge of combat training for the National Guard, was among the dead.

The helicopter's downing came amid reports of intensive gunfire outside the city on May 29.

Russia's Foreign Ministry repeated its demand that the Ukrainian government stop its military operation against pro-Russian separatists and urged the United States and European Union to put pressure on Kyiv to avert a disaster.

"We once again demand that the Kyiv authorities stop the fratricidal war and start a real national dialogue with all political forces and representatives of the country's regions," a ministry statement said.

"We again call on our Western partners to use all their influence on Kyiv to stop Ukraine's slide into national catastrophe," it added.

Slovyansk, in the Donetsk region, has seen fierce fighting in recent weeks as government forces continue their offensive against rebels in the east of the country.

Pro-Russian separatists in Donetsk and the neighboring Luhansk region have declared their own independent "people's republics."

On May 29, the self-declared separatist "mayor" of Slovyansk said his fighters were holding four OSCE observers who went missing earlier this week.

Vyacheslav Ponomaryov promised the civilian observers would be freed soon but also suggested they were suspected of spying.

In the Luhansk region, separatists on May 28 raided a barracks of the volunteer National Guard.

Both sides say they suffered losses in the fighting.

Petro Poroshenko, the winner of Ukraine's presidential election, who takes office early next month, has vowed to continue what the government describes as an "antiterrorist operation" and to bring the crisis in the east to a quick end.

Poroshenko also said he was ready to hold talks with Russian President Vladimir Putin at next week's D-Day anniversary commemorations in France.

White House Press Secretary Jay Carney said on May 29 that the Obama administration was "deeply disturbed" by the reports, though he said that they could not verify the "details" of them.

Prime Minister Arseniy Yatsenyuk said on May 28 that if Russia stopped supporting the separatists, the crisis in the east would be resolved "in a week or two."

Russia denies it is behind the armed separatist insurgency, saying the rebels are defending the interests of Russian-speakers in the east.

Russian Foreign Minister Sergei Lavrov on May 29 called for "more efficient mediation efforts" to stop the violence.